The present invention is directed to methods and apparatus for preserving fluid foodstuffs, and more particularly is directed to such methods and apparatus for extending the shelf life of perishable fluid foodstuffs such as dairy products, fruit juices and liquid egg products, which are growth media for microorganisms. The present invention is also directed to preserved liquid foodstuffs which have extended shelf life.
Substantial technical effort has been directed to the preservation of perishable fluid food products such as milk products, natural fruit juices and liquid egg products which may normally contain a wide variety of microorganisms, and which are excellent culture media for microorganisms.
Practical preservation methods which have found significant commercial application predominantly utilize heat treatment such as pasteurization to inactivate or reduce the microorganism population. For example, milk products are conventionally pasteurized at a minimum temperature of at least about 72.degree. C. for 15 seconds (or equivalent time/temperature relationship) to destroy pathogenic bacteria and most of the nonpathogenic organisms, with degradative enzyme systems also being partial or totally inactivated. However, products processed in this manner are still generally nonsterile and have limited shelf life, even at refrigeration temperature. The shelf life of liquid foodstuffs may be substantially extended by higher heat treatment processes such as "ultra high pasteurization", or "ultra high temperature"("UHT") such as treatment of from about 94.degree. C. for 3 seconds to about 150.degree. C. for one second in conjunction with aseptic packaging to achieve complete destruction of all bacteria and spores. However, such heat treatment typically adversely affects the flavor of the food product, at least partially denatures its protein content or otherwise adversely affects desired properties of the fluid food product. Other approaches to liquid food preservation, which also have certain disadvantages, include the use of chemical additives or ionizing radiation.
The bactericidal effects of electric currents have also been investigated since the end of the 19th century, with various efforts having been made to utilize electrical currents for treating food products, such as described in U.S. Pat. Nos. 1,900,509, 2,428,328, 2,428,329 and 4,457,221 and German Patents 1,946,267 and 2,907,887. The lethal effects of low-frequency alternating current with low electric field strength have been largely attributed to the formation of electrolytic chemical products from the application of current through direct contact electrodes, as well as ohmic heating produced by current flow through an electrically resistive medium. As described in U.S. Pat. No. 3,594,115, lethal effects of high voltage arc discharges have also been attributed to electrohydraulic shock waves. However, such electrolytic chemical products may be undesirable in fluid foodstuffs, and the utilization of explosive arc discharges to produce microbiologically lethal shock waves has not found wide-spread application in the provision of edible liquid foodstuffs having extended shelf life.
More recently, separately from the art of food preservation, the effect of strong electric fields on microorganisms in nonnutrient media has been studied as a mechanism for reversibly or irreversibly increasing the permeability of the cell membrane of microorganisms and individual cells [Sale, et al., "Effects of High Electric Fields on Mi Lysis of Erythrocytes and Protoplasts", Biochimica et Biophysica Acta, 163, pp. 37-43 (1968); Hulsheger, et al., "Killing of Bacteria with Electric Pulses of High Field Strength", Radiat. Environ. Biophys; 20, pp. 53-65 (1981); Hulsheger, et al., "Lethal Effects of High-Voltage Pulses on E. coli K12", Radiat. Environ. Biophys. 18, pp. 281-288 (1980); Zimmermann, et al., "Effects of External Electrical Fields on Cell Membranes", Bioelectrochemistry and Bioenergetics, 3, pp. 58-63 (1976); Zimmermann, et al., "Electric Field-Induced Cell-to-Cell Fusion", J. Membrane Biol., 67, pp. 165-182 (1982); Hulsheger, et al; "Electric Field Effects on Bacteria and Yeast Cells", Radiat. Environ. Biophys; 22, pp. 149-162 (1983); U. Zimmermann, et al., "The Development of Drug Carrier Systems: Electrical Field Induced Effects in Cell Membranes", Biochemistry and Bioenergetics, 7, pp. 553-574 (1980); Jacob, et al., "Microbiological Implications of Electric Field Effects II. Inactivation of Yeast Cells and Repair of Their Cell Envelope", Zeitschrift fur Allgemeine Mikrobiologic, 21, 3, pp. 225-233 (1981); Kinositas, Jr., "Formation and Resealing of Pores of Controlled Sizes in Human Erythrocyte Membrane", Nature, 268, 4, pp. 438-440 (Aug., 1977); Neamann, et al., "Gene Transfer into Mouse Lyoma Cells by Electroporation in High Electric Fields", IRI Press Limited, Oxford, England, pp. 841-845]. The application of high electric fields to reversibly increase the permeability of cells has been used to carry out cell fusion of living cells and to introduce normally excluded components into living cells. Electric fields in nonnutrient media have a direct lethal effect upon microorganisms with the rate of kill dependent upon the field strength above a critical field level and the duration of the applied high voltage pulse or pulses.
These studies postulate the cell membrane as the site of a critical effect, of reversible or irreversible loss of membrane function as the semipermeable barrier between the cell and its environment. An external field of short duration is assumed to induce an imposed transmembrane potential above a critical electric field value, which may produce a dramatic increase of membrane permeability. Because an increase in cell permeability prevents the counteracting of differences in osmolality of the cell content and surrounding media, exchange or loss of cell contents, cell lysis and irreversible destruction may occur as secondary mechanisms in nonnutrient media which limit the ability of cells to repair themselves, and which adversely affect permeable cells through osmotic pressure differences between the medium and the interior of the cell.
U.S. Pat. No. 4,838,175 and U.S. Pat. No. 4,695,472 identified hereinabove, are directed to methods and apparatus for providing fluid food products having extended shelf life. However, improved methods and apparatus of this type would be desirable, particularly such methods and apparatus having improved performance and efficiency. Accordingly, it is an object of the present invention to provide methods and apparatus for extending the shelf life of perishable pumpable food products such as dairy products, natural fruit juices, pumpable fluid egg products, beer, wine, soups, stews, gravies, particulate food suspensions or slurries and other pumpable food products. These and other objects of the present invention will become apparent from the following detailed description and the accompanying drawings.